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Concerning reference frames

  1. Sep 2, 2010 #1
    Hey guys I don't think i have the correct solution to this question. I would say tht the laser beams would bothe reach the planet at the same time, but this just seems way too easy. Any hints/thoughts/suggestions on solutions to this question? Thanks

    Two space craft are flying towards a planet at 0.9c and 0.3c relative to the planet, respectively. If they fire their laser beams simultaneously from a range of 109 m, which will strike the planet first and what will be the delay before the other beam strikes?
  2. jcsd
  3. Sep 2, 2010 #2
    It might be too easy, but it's true. The laser beams travel at the speed of light, which is independent of coordinate frame.
  4. Sep 2, 2010 #3
    yeah perhaps they r looking for a bit of an explanation as to why this would be so. thanks for helpin me out man :D
  5. Sep 2, 2010 #4
    Oh this ones easy. And experimentally verified a million times over.

    First if they are both equal distance from the planet, the signals will reach the earth at the exact same time. They both however will blueshift (which I hope links to wiki) with the 0.9c craft blue shifting more than the 0.3c. f+=sqrt((1+v2/c2)/(1-v2/c2)). f-=sqrt((1-v2/c2)/(1+v2/c2)). And that's from memory!
  6. Sep 2, 2010 #5

    Doc Al

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    Staff: Mentor

    This problem must be making the rounds. (See: https://www.physicsforums.com/showthread.php?t=425249)

    As I said in the other thread, the problem seems ambiguously worded. (Unless they just want to test if you realize that the speed of the beams is independent of the speed of the ships.) If this is a textbook problem, what textbook?
  7. Sep 2, 2010 #6


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    Staff Emeritus
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    I'll quote myself:
    Some of the other posts in that thread might be useful too. The > above the quote is a link.
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